Default replication is asynchronous. Meaning master writes to binary log and the slave connects and “pulls” contents of the binary log. Bad thing is if the master crashes, there’s no guarantee that a slave has all committed transanction.

Simi-Synchronous Replication is an alternative to asynchronous replication. Midway point between asynchronous and fully syncronous. Master only waits for a slave to receive an event. Don’t have to wait for slaves to actually commit.

Performance schema tracks at an extremely low level. Just like Information schema, tables are views or temporary tables. Activation doesn’t cause any change in server behavior. This is designed for advanced users.

Think of SIGNAL as an exception, a way to “return” an error. You get exception-handling logic for stored procedures, stored functions, triggers,events and db apps.

RESIGNAL lets you pass error information up. Think of it as a catch. Requres an active handler to execute. Lets you program on your PHP side to catch that very specific handling.